Levulinate esters are known to be useful as plasticisers and solvents and have been suggested as fuel additives. Levulinic acid can be obtained by acid hydrolysis of cellulose-containing biomass or sugars derived therefrom. Such acid hydrolysis processes are known in the art, for example from WO 89/10362, WO 96/40609, U.S. Pat. No. 5,892,107 and U.S. Pat. No. 6,054,611. Such acid hydrolysis processes yield an aqueous mixture comprising levulinic acid, formic acid, furfural (if C5-sugars containing hemicelluloses were present in the starting material), and the mineral acid that was used as catalyst for the hydrolysis. In the art, several processes for the preparation of levulinate esters starting from such aqueous mixture obtained by acid hydrolysis of biomass are described.
In U.S. Pat. No. 2,029,412 is disclosed the preparation of 2-methylbutyllevulinate ester by esterifying a concentrated, levulinic acid containing syrup obtained from acid hydrolysis of cane sugar with 2-methylbutanol. Water is removed during the esterification process. After the esterification process has been stopped, alcohol is distilled off. The ester is recovered by vacuum distilling the remaining mixture. In the process of U.S. Pat. No. 2,029,412, the amount of water present during the esterification process is very low, since the starting material is a concentrated syrup and water is removed during the esterification process. Disadvantages of the process are that a large amount of energy is needed for water evaporation and that the mineral acid will remain in the product ester stream.
In WO 98/19986 is disclosed the preparation of a levulinate ester by adding methyl or ethyl alcohol to an aqueous levulinic acid/sulphuric acid mixture and refluxing the resulting mixture. The amount of alcohol is in stoichiometric excess to the amount of levulinic acid. It is mentioned that the levulinate ester can be recovered by phase separation after the excess alcohol is distilled off. Separation of the ester from the resulting mixture by means of chromatography is also mentioned.
In WO 97/47579 is disclosed a process for the separation of levulinic acid from a reaction mixture of water-soluble components wherein the levulinic acid is first esterified with an alcohol to produce a water insoluble ester. The ester is then separated from the reaction mixture and subsequently hydrolysed to yield the acid and the alcohol. The alcohol is present in stoichiometric excess to the amount of levulinic acid. The formation and hydrolysis of methyl levulinate is exemplified.
In GB 1,282,926 is disclosed a process wherein an aqueous, levulinic acid-containing solution is contacted with a water-miscible esterifying solvent to form an esterifying mixture. The esterifying mixture is simultaneously contacted with a water-immiscible organic solvent to extract the esters formed. The water-miscible esterifying solvent is preferably a lower alkyl alcohol having one to five carbon atoms and the water-immiscible organic solvent is preferably benzene or chloroform.
In WO 03/085071 is disclosed a process for the preparation of a mixture comprising levulinic acid esters and formic acid esters from biomass, wherein a reaction mixture comprising levulinic acid and formic acid is contacted with an olefin to form an organic phase containing the levulinic acid esters and formic acid esters and an aqueous phase. The olefin is preferably contacted with the reaction mixture in the presence of a water-immiscible hydrocarbon solvent.
The prior art processes of WO 98/19986, WO 97/47579, GB 1,282,926 and WO 03/085071 have several disadvantages. In the processes such as disclosed in WO 98/19986 and WO 97/47579, an aqueous reaction mixture is obtained that contains a relatively high concentration of organic compounds including furfural. As a consequence, the aqueous mixture has to be treated before it could be recycled, since the presence of furfural in the acid reaction mixture may result in the formation of undesired, tar-like by-products. In the processes as disclosed in GB 1,282,926 and WO 03/085071, the esters are extracted from the reaction mixture during or after esterification by means of a water-immiscible solvent. In these processes both an esterifying agent and an extracting solvent are used. This means that both the solvent and the excess of esterifying agent have to be removed from the product streams if the esters are to be obtained in pure form.